You must be either over 16 years old or don't have kids or don't work in a high school. This really DOES solve a problem.
The problem is, say a 15 year old girl texts her best friend or boyfriend and 3 or 4 minutes goes by and she does not get a reply. After the 5th minute she is certain the other person hates her and the relationship is over and her life is ruined and she is going to be a social outcast and so on and so on.
This let's the other person send a "yo" back which can done very quickly with zero thinking or typing.
There is a similar problem with some that goes the other direction. NOT getting a message for a long time (like a couple hours) means the same as not getting a reply in 4 minutes. They rarely care much what the message says so "yo" is as good as anything else.
It's hard for us old folks (those over 30) to understand but I've seen this kind of thing many times. The kids ARE smart enough to know that they might not get an instant reply during class time or whatever or at 3:00am but no instant return when he could have sent one is a big deal to very insecure people, like teens.
This is the most insightful post I have ever read on this forum. Thanks!